While working on reproducible builds for openSUSE, I found that
since the introduction of rpm-4.18 3 weeks ago,
many Factory packages now vary from parallelism in their .src.rpm headers.
e.g. building with osc build -j3 and -j4 gives 

openSUSE:Factory/abcm2ps # filterdiff 'strings' RPMS*/*src.rpm

--- strings RPMS.2/abcm2ps-8.14.14-0.0.src.rpm
+++ strings RPMS/abcm2ps-8.14.14-0.0.src.rpm
@@ -1,6 +1,7 @@
 abcm2ps-8.14.14-0.0
- 7f401a1d6b1de68d118627417c967c5a3d3bcdb8
-e2020445b31966aa351b789c86275831a3121502e6fcf5b550a1caa835313886
+ c4291b1c9ffe3ecbfad5785cfd63e7574158e1fa
+12af42be780406c7ed6eaef3cec3f885fa5d3b7bb36269cdbb2fb34e888c8a4c
+p|oK
 abcm2ps
 8.14.14
 A program to typeset abc tunes into Postscript
@@ -334,7 +335,7 @@
        --sharedstatedir=/var/lib \
        --mandir=/usr/share/man \
        --infodir=/usr/share/info
-make -j3
+make -j4
 %install
 make install
DESTDIR="/home/abuild/rpmbuild/BUILDROOT/abcm2ps-8.14.14-0.0.x86_64"


It seems to capture the expanded .spec file in rpm headers. Especially
  make %{?_smp_mflags}

Binary rpms vary as well, because these contain a .src.rpm hash.

This is different from earlier bug 1197780.